head.h:
#ifndef HEAD_H
#define HEAD_H
int add(int,int);
int sub(int,int);
int mul(int,int);
double div(int,int);
#endif

add.c:
#include "head.h"
int add(int a,int b){
return a+b;
}

sub.c:
#include "head.h"
int sub(int a,int b){
return a-b;}

mul.c:
#include "head.h"
int mul(int a,int b){
return a*b;
}

div.c:
#include "head.h"
double div(int a,int b){
return a/b;
}

main.c:
int main(void){
int a,b;
printf("enter two numbers:");
scanf("%d %d",&a,&b);
printf("add:%d\n",add(a,b));
printf("sub:%d\n",sub(a,b));
printf("mul:%d\n",mul(a,b));
printf("div:%d\n",div(a,b));
return 0;
}
